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https://tc39.github.io/ecma262/#sec-array-exotic-objects seems to define array index somewhat informatively. Could there be a more formal definition there so it can be reused by IDL?
You probably also do not want to define it only for String properties, since as far as I can tell https://tc39.github.io/ecma262/#sec-array-exotic-objects-defineownproperty-p-desc can invoke it upon a Symbol, for which it would not be defined?
Something like an IsArrayIndex abstract operation would be great.